Scottish Government

Fisheries Missions - North Sea Herring Survey

• Mission Type F1: Precis ion Enhancement Study (four missions)
  Autosub-1 will be directed to a transect mid-way between the present and subsequent RV transects. It would travel along this transect for 16 hours (115 km at 2 m/s (~4 knots)) near-surface, collecting data from downward looking transducers of exactly the same format as that being collected by the RV. Autosub-1 would then surface at a predetermined point (maintaining station by GPS) on the RV transect for recovery.
• Mission Type F2: Surface Observations (three missions)
  These would be of similar duration to Type 1 (16 hours = 115 km at 2 m/s) but depth of operation would be greater (100 m) and the transducers would be orientated towards the surface. Mission Types 1 and 2 require D-cell batteries for their long ranges.
• Mission Type F3: Avoidance Study (two missions)
  These would be similar to Type 3 (eight hours), but with two notable differences: Autosub-1 would be at the near-surface looking down in the more established manner to be certain of quantification; and the distance ahead of the RV would be altered hourly to give replicates of AUV/RV separations of 200, 400, 600 and 1,000 metres. This experiment will be aided (for example in correcting for temporal variability at larger distances) by the qualitative data on herring distribution available from the RV's research SONAR (Simrad SF 950). These missions will take place in an area where the abundance of fish is expected to be high. Mission Types 3 and 4 require lead-acid batteries.

Antarctic Missions

• Mission A1
  When approaching the ice edge, it is usual to pass through bands of loose sea ice separated by 1,000's of metres. Mission 1, 11 km in length, would pass beneath one or two of these bands. Autosub-1 would be deployed in open water, dive to 200 m and proceed for 5 km in a direction normal to the ice bands. At 200 metres Autosub-1would be well clear of any downward-projecting ice keels, but the sea-surface/ice ceiling would be well within detection range of the 38 and 120 kHz signals. After 5 km Autosub-1 would turn 90° to port, run for 1 km ascending to 150 metres, turn a further 90° to port, and proceed for 5 km before surfacing. RRS James Clark Ross is ice strengthened and would be able to follow Autosub-1's track, thus maintaining acoustic contact. On transect, the upward looking acoustic system, configured with both transducers pinging in synchrony, would collect data of exactly the same format acquired by BAS during surface-run acoustic surveys of krill biomass. In addition, the 38 kHz beam (7.1 degrees wide) with an acoustic footprint of 24.8 metres diameter at a range of 200 metres, would obtain independent measurements of ice thickness at a horizontal resolution of approximately 25 metres. This mission offers a first, very-low risk, deployment of Autosub-1 under ice, and would provide data on thickness and bottom topography of sea-ice and relative abundance of krill in open water and under ice bands.
• Missions A2 and A3
  Autosub-1would dive to 200 metres and head towards the ice edge, passing beneath ice bands and on under consolidated pack. The outward leg would be 20 km long (approximately three hours). After 20 km Autosub-1 would turn 90° and proceed in a direction roughly parallel to the ice edge for 1.5 hours (approximately 8 km - transect spacings are of this order for BAS surveys of krill biomass in open water - Brierley et al., 1997), ascending to 150 metres throughout this leg (giving horizontal resolution of 20 metres). The vehicle would then turn a further 90°, placing it on a parallel reciprocal course to the outward leg, and proceed to open water beyond ice bands for recovery. This mission (48 km, seven hours, max depth 200 metres) is well within the present quoted endurance and operational limits of the Autosub-1 operating on lead acid batteries (leaves a range reserve of 20%). It will collect data on krill distribution and aggregation pattern under ice and in open water, and on mesoscale variability in ice thickness at two horizontal scales. In addition, the ADCP data set will be used to assess the possibility of using "bottom tracked" data from the ice ceiling for navigation. Mission 3 would begin 9 km along the ice edge from the point of recovery at the end of Mission 2, extending areal coverage.
• Mission A4 and A5
  Of the same design as 2 and 3, these deployments would collect further data on the distribution and abundance of krill. The outward leg would, however, be at a depth of 100 m and the return at 50 metres. Both would collect ice thickness data at a further two scales of resolution (15 m and sub-10 metres). At 50 metres depth, the return leg would, in addition, enable measurement of downwelling irradiance (by PAR sensor). These irradiance data would then be linked to acoustic estimates of ice thickness.
• Mission A6 and A7
  Again of the same track design, but to be run over day/night interface to investigate diurnal variation in behaviour and distribution of krill under ice. Autosub-1 would travel quite deep to allow insonification of upper 150 metres of water column, the area where krill are seen in open water and where those limited acoustic observations that have been made under ice show some krill to be.
• Mission A8/9
  Avoidance study in open water, to be based on protocols/results from the North Sea study.
• Mission A10
  Beneath tabular icebergs. The target iceberg would be chosen to have a freeboard of less than 20 metres, this would give a maximum expected draft of less than 100 metres (Weeks and Mellor, 1978). We would deploy Autosub-1 2 km from the iceberg, fly the vehicle at 200 metres depth beneath the iceberg, on for another 2 km, turn and fly back beneath the iceberg for recovery. This mission would provide insight into the bottom topography of an iceberg, and investigate whether enhanced biological activity (see Kaufman et al., 1995) can be observed. This mission would be repeated if time permitted on a number of icebergs.